Lateral bores are frequently needed to improve or continue production from a wellbore. When the drilling of a lateral becomes necessary, a diverter tool with a long ramp known as a whipstock is run in and oriented and anchored. Usually, to save rig time, an assembly of mills is run in with the whipstock. The anchor for the whipstock can be run in separately or together with the whipstock. The whipstock ramp is at a very gradual angle to slowly guide a window mill into the casing wall to make an opening known as a window. Generally, the initial mill is secured for run in to the top of the whipstock ramp at a lug. Initiating window mill rotation breaks the lug connection and allows the window mill to descend. As the window mill descends it is guided by the ramp that is sloped at about 3 degrees slowly into the casing wall. As a result, the initial window shape is somewhat elliptical. It starts fairly narrow and gets as wide as the window mill. The window gradually narrows as the mill moves its centerline past the casing wall while continuing to descend to the bottom representing a complete exit of the window mill through the casing wall.
For a variety of reasons the width or diameter of the window made by the window mill is not the desired final window diameter required to support the exit of other tools to drill and complete the lateral (e.g. a window must be able to pass or ‘drift’ a tool with an outside diameter larger than normal). Typically, the window mill is run in tandem with other mills disposed uphole from it. These generally elliptically shaped mills have come to be known as watermelon mills because of their appearance. Typically the watermelon mill(s) get the same size as the window mill or get progressively larger in the order that they enter the initial window so that the window can be made wider in stages. This type of system done in a single trip is shown in U.S. Pat. No. 5,109,924.
When sizing the watermelon mills attention has to be paid to the spacing of the mills, the degree of flexibility of the connecting pieces and the inside clearance of the tubulars through which the watermelon mills must pass, to name a few considerations. Many times the inside clearance diameter in the tubular, known as the drift diameter is the limiting factor on the maximum dimension of the watermelon mill. Clearly, if the watermelon mills can be made bigger and still reach the window, they would be in a better position to enlarge the window to at least the minimum needed width for drilling and completing the lateral through the window. A specific application would be the installation of an expandable liner through the window then allowing a constant ‘drift’ through the original wellbore casing as well as the new lateral liner.
In drilling applications, under-reamers have been used that have blades that retract and then get pivoted by one end to go to a bigger size for increasing the bore size. The opposite end from the pivot end is cantilevered. Other designs for a technique called reaming while drilling (RWD) use reaming blades that are telescoped radially to make a bigger hole than the bit that is adjacently mounted. Some recent under reaming tools are illustrated in U.S. Pat. Nos. 7,048,078; 6,920,944 and 6,880,650.
The present invention addresses this concern by providing a watermelon mill that can pass a given drift diameter in the tubular and then have its outside dimension increased shortly before or during the time it reams the window made by the window mill wider. With the ability to increase the milling diameter, a window with a larger drift than the original wellbore casing can be milled. Also, the number of mills in a bottom hole assembly can be reduced and that makes it easier to run the assembly in a well particularly if it is highly deviated. The diameter of the mill can be changed even as it reams the window. These and other features of the present invention will be more readily understood by those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while the claims define the full scope of the invention.